1. Field of the Invention
The present invention relates to a method of aligning first and second objects relative to each other and a device for the same, and more particularly, it relates to a method and device for aligning a mask (reticle) and a wafer relative to each other when the image of a circuit pattern is to be transferred onto the wafer in the course of manufacturing semiconductors, for example.
2. Description of the Related Art
The projecting and exposing apparatus used in the course of manufacturing semiconductors such as LSI includes a mask (reticle) on which a circuit pattern is previously formed and a wafer on which the image of this circuit pattern is transferred. As shown in FIG. 1, wafer 2 is mounted on wafer table 3, which is moved on base 4 by a driver means (not shown). Mask or reticle 1 is arranged above wafer 2. Projecting lens 5 is arranged between reticle 1 and wafer 2. When the circuit pattern previously formed on reticle 1 is irradiated by exposing light, therefore, the image of the circuit pattern is reduced and transferred on wafer 2 through projecting lens 5. It is needed before this transferring process that reticle 1 and wafer 2 are accurately aligned relative to each other.
This process of aligning the reticle and the wafer relative to each other includes a first step (reticle alignment) of aligning reticle 1 and wafer table 3 relative to each other and a second step of aligning reticle 1 and wafer 2 relative to each other.
The first step will be described referring to FIGS. 1 through 4. This alignment includes rough and fine ones. A mark and an ITV camera are used for the rough alignment. A mark and an observation optical system are used for the fine alignment. As shown in FIG. 2A, reticle 1 is provided with mask or reticle mark 8 having two windows 6 and bar 7. As shown in FIG. 2B, wafer table 3 is provided with reference mark 11 having two reflecting faces 9 and bar 10. Observation optical system 12 is located on one side of reticle 1, as shown in FIG. 1. This optical system 12 includes photo-sensor 13, vibration slit 14, vibrator 15, lenses 16, and mirror 17. Optical system 12 detects reticle and reference marks 8 and 11 according to the vibration slit method.
Reticle and reference marks 8 and 11 are observed by ITV camera 18 in the case of the rough alignment. Namely, images of bar 7 and bar 10 projected in window 6 are observed by ITV camera 18, as shown in FIG. 2C. Reticle 1 and wafer table 3 are moved and bars 7 and 10 are matched relative to reference line 19 of optical system 12, respectively. Reticle 1 and wafer table 3 are thus aligned relative to each other, as shown in FIG. 1.
Reticle 1 and wafer table 3 are then minutely aligned relative to each other, as shown in FIGS. 3 and 4. Reticle 1 is temporarily moved left from its position where it is to be aligned, as shown in FIG. 3. Reference mark 11 is matched relative to reference line 19 of optical system 12. More specifically, when reference mark 11 is irradiated by light, the image of bar 10 of reference mark 11 is projected on vibration slit 14 via projector lens 15, mirror 17 and lens 18. When wafer table 3 is shifted by u in position, for example, image 20 of bar 10 is projected on a position which is separated from reference line 19 by k.multidot.u. Amount u of wafer table 3 shifted is therefore proportional to the distance k.multidot.u between image 20 of bar 10 and reference line 19. Vibration slit 14 is being vibrated at a certain frequency by vibrator 15, taking reference line 19 as its vibrating center. When the position of opening 21 of vibration slit 14 is in accordance with that of image 20 of bar 10, therefore, image 20 of bar 10 is entered into photo-sensor 13 through opening 21 of vibration slit 14. Distance k.multidot.u between image 20 of bar 10 and reference line 19 is calculated basing on signals of image 20 detected by photo-sensor 13 and certain frequency f. Amount u of wafer table 3 shifted is obtained from this distance k.multidot.u. Therefore, bar 10 of reference mark 11 is aligned relative to reference line 19, moving wafer table 3 to make this position-shifted amount u zero. Coordinates of reference mark 11 on wafer table 3 are read this time according to the laser interference manner.
Reticle mark 8 is then matched relative to reference line 19 of optical system 12, as shown in FIG. 4. Wafer table 3 is moved from that position where it is to be aligned, to thereby allow light, which is reflected by wafer table 3, to irradiate reticle mark 8. The light passed through reticle mark 8 is introduced to vibration slit 14 via mirror 17 and lenses 16. The image of bar 7 of reticle mark 8 is thus projected on vibration slit 14. Bar 7 and reference line 19 are therefore aligned relative to each other, similarly to the case where bar 10 of reference mark 11 is aligned relative to reference line 19. Wafer table 3 is then returned to that position where it is to be aligned, basing on read values of the coordinates of wafer table 3. Reticle 1 and wafer table 3 are thus aligned relative to each other, as shown in FIG. 1.
However, reference mark 11 and reference line 19 of optical system 12 are aligned relative to each other, reticle mark 8 and reference line 19 are then aligned relative to each other, and as the result, reticle 1 and wafer table 3 are aligned relative to each other, as described above. Therefore, the conventional method has two aligning processes, thereby becoming complicated in the alignment. Further, the alignment is achieved by two processes, thereby making it necessary to move the reticle and the wafer table in the course of alignment. The conventional method is therefore likely to cause error in the alignment.
When the voltage for vibrator 15 is changed by some reason, the vibrating center of vibration slit 14 is drifted. That is, reference line 19 is drifted. Error is therefore sometimes caused in aligning reticle 1 and wafer table 3 relative to each other.
Furthermore, it is needed that reference line 19 is arranged perpendicular to lenses 16 and the reticle mark, for the purpose of accurately projecting the images of bars 7 and 10 of reticle and reference marks 8 and 11 onto vibration slit 14 without causing any error in their magnification. In the case where lenses 16 are single-sided telecentric, therefore, luminous flux passed through reticle mark 8 is introduced to vibration slit 14, shifting from and passing above reference line 19. Namely, this luminous flux passes through the upper halves of lenses 16. It is therefore needed that lenses 16 and mirror 17 are made large in size. As the result, optical system 12 becomes large in size.
Still further, it is needed that the aberration of lenses 16 is corrected, in order that the luminous flux passed through reticle mark 8 can pass through the whole of lenses 16. This makes the arrangement of the lenses complicated.
The second process of aligning the reticle and the wafer relative to each other will be described.
As shown in FIG. 5, wafer 2 is provided with wafer mark 22 similar to reference mark 11. However, reticle 1 and wafer 2 cannot be directly aligned relative to each other. Therefore, optical system 23 (or off-axis system) different from above-mentioned optical system 12 is located adjacent to one side of wafer table 3. Optical system 23 includes photosensor 13, vibration slit 14, vibrator 15 and the like, similarly to optical system 12. Wafer table 3 is moved and wafer mark 22 is matched relative to reference line 24 of optical system 23. Coordinates of wafer table 3 are read at this position. Wafer table 3 is then moved and reference mark 11 of wafer table 3 is matched relative to reference line 24 of optical system 23. Coordinates of wafer table 3 are read at this position. The positional relation between reference mark 11 and wafer mark 22 is thus obtained. When wafer table 3 is moved a certain distance, therefore, wafer mark 22 is aligned relative to reticle mark 8.
The alignment of the reticle and the wafer is carried out by two steps also in the second process. Therefore, error can be caused in the alignment. Similarly to optical system 12, the vibrating center of vibration slit 14 is drifted to thereby cause error in the alignment. Because the second process is carried out using the off-axis system in which the light detected is not passed through the projector lens, it is more likely to cause error in the alignment, as compared with the first process (or TTL system) in which the light detected is passed through the projector lens.